URL Structure: Lower case VS Upper case

I'm going to have to disagree with all established wisdom on this, so I'll probably get downvoted, but:

If you redirect all mixed case urls to your properly cased url, it solves all the problems mentioned. Therefore it seems this argument is coming from tradition and preference. The point of a URL is to have a user-friendly representation of a page, and if your url is friendlier with upper case, why not use it? Compare:

moviesforyoutowatch.com/batman-vii-the-dark-knight-whatevers MoviesForYouToWatch.com/Batman-VII-The-Dark-Knight-Whatevers

I find the mixed case version superior for the purpose. If there's a technical reason that can't be solved with a lower-case compare and redirect, please share it.

The domain part is not case sensitive. GoOgLe.CoM works. You can add uppercase as you like, but normally there's not a reason to do so and, as stated in the comments below, may hurt your SEO ranking.

The path part is or is not case sensitive, depending on the server environment and server. Typically Windows machines are case insensitive, while Linux machines are case sensitive. This means that you should stick to lowercase or you risk introducing a bug that's really hard to hunt down (mismatched case that doesn't matter on the dev server).

The query string part is available to the server as it is. You can readily use mixed-case as you like, or discard the case (toLowerCase(...)). This also means that using a base64-encoded keys will work. You can't expect the users to type that correctly, though.

The hash part (called "fragment identifier") is only available to the client code, not to the server. Javascript may distinguish between the cases as it likes, and so does the browser. url#a will scroll to the element with the ID a, but url#A won't.

I know you asked for technical reasons but it's also worth considering this from a UX perspective.

Say you have a URL with upper case characters and, for arguments sake, this has been distributed on printed media. When a user comes to enter that URL into their browser they may well be compelled to match that case (or be forced to match the specified case if your web server is case sensitive) ultimately you are giving them more work to do as they have to consider case as well. After all, they don't know if your server is case sensitive or not and they may have experienced 404s from case sensitive web servers in the past.

If your server is case sensitive and you are using mixed case URLs you are giving more scope for the user to mistype the URL. Furthermore, say you have the URL www.example.com/Contact. It's easy to confuse an upper and lower case "c" (especially if it is copied in hand writing) if the user overlooks this and uses the wrong case they may never reach your content.

With all this in mind consider www.example.com/News/Articles/FreeIceCreamForAll. On keyboard that's not too difficult but consider this on a mobile device, it would be very fiddly to input.

The reverse is also true should a user want to write down a URL from the address bar. They may feel they need to match the case, ultimately giving them more work to do and increasing the likelyhood of errors.

To conclude; keep URLs lower case.